A great variety of thermal recording materials and processes, which are based on the application of heat or other high intensity radiation to selected portions of thermosensitive media, are known in the art. Many of these processes entail diffusion transfer, whereby a color-producing substance is transferred from a heated medium to an image-receiving layer. U.S. Pat. No. 4,892,602, for example, discloses thermally promoted transfer of a colorant from a metal deposition layer to plain paper. Similarly, U.S. Pat. No. 5,273,808 describes thermal transfer media containing inorganic and organic pigments and dyes as colorants.
In contrast to thermal transfer, a direct thermal recording process produces a desired image in a heat-sensitive material, with no subsequent transfer step. Direct thermal recording requires simpler processing equipment than transfer imaging; it is also ecologically advantageous in that it does not generate by-product materials requiring disposal.
Direct thermal image recording materials find widespread use in varied applications. They can be employed, for example, with infrared copying machines to produce images on transparent or translucent supports such as plastic films or on opaque supports such as paper, as disclosed in U.S. Pat. Nos. 4,551,738; 5,079,212; 5,296,440; and 5,424,182, the disclosures of which are incorporated herein by reference.
Other applications of direct thermal image recording include media used in cash registers, calculators, scientific instruments, and the like, as well as various sorts of tickets and tags. A particularly important application is the printing of adhesive labels, as described in U.S. Pat. Nos. 4,370,370; 4,570,169; and 4,886,774, the disclosures of which are incorporated herein by reference.
Various imaging chemistry systems have been utilized in direct thermal image recording materials. The use of mono-, oligo-, and polysaccharides together with a catalyst such as a protic or Lewis acid is described in U.S. Pat. No. 5,472,930, the disclosure of which is incorporated herein by reference. An image-producing combination of a noble metal salt of an organic acid such as silver behenate and an inorganic reducing agent such as a gallic acid ester is disclosed in the previously cited U.S. Pat. No. 5,424,182. A very frequently employed direct thermal imaging chemistry system entails the use of a colorless or pale-colored dye precursor, or leuco dye, with an acidic dye-forming agent such as a phenolic compound. Materials utilizing leuco dyes are described in the aforementioned U.S. Pat. Nos. 4,370,370; 4,551,738; 4,570,169; 4,886,774; 5,079,212; and 5,296,440, the disclosures of which have been incorporated herein by reference.
Thermal recording materials are frequently subject to conditions that impair the permanence of the recorded images. In some instances, subsequent erasure of an image may be desired, as with the reversible thermosensitive coloring compositions described in U.S. Pat. No. 5,432,534. More often, however, enhancing the stability of recorded images is sought. Thermal recording materials often exhibit poor resistance to physical abrasion. In addition, the recorded images may suffer photodegradation by ultraviolet radiation, a problem discussed in the previously mentioned U.S. Pat. No. 4,886,774. Images may also be attacked by moisture or other agents such as plasticizers that diffuse into recording material through either its front or rear surface. Protective overcoats or barrier layers for direct thermal recording materials are described, for example, in the previously mentioned U.S. Pat. Nos. 4,370,370; 4,551,738; 4,570,169; 5,079,212; 5,296,440; and 5,424,182.
Embodiments of the present invention provide thermosensitive direct image-recording materials having high resistance to abrasion and other physical damage as well as to ultraviolet and chemical degradation of the recorded images.